docs: adjust script to handle zero unused images DOC-1232

.github/workflows/clean-up-unused-images.yaml

@@ -5,6 +5,8 @@ on:
     # On the first of every month at 2 am
     - cron: '0 2 1 * *'
   workflow_dispatch:
+  push:
+    branches: ['handle-zero-images-doc-1232']
 
 concurrency:
   group: clean-up-images-${{ github.ref }}
@@ -49,10 +51,16 @@ jobs:
 
       - name: Create PR with unused images
         run: |
-          unused_image_count=$(wc -l < unused_images.json)
-          if unused_image_count == 0; then
-            "No images found to remove."
-            exit 0
+          file="unused_images.json"
+          if [[ -f "$file" ]]; then
+            unused_image_count=$(wc -l < "$file")
+            if $unused_image_count == 0; then
+              "No images found to remove."
+              exit 0
+            fi
+          else
+            echo "File $file not found."
+            exit 1
           fi
 
           # Ensure that we are on master.